A_Dictionary_of_Chemical_Engineering_Car

Prévia do material em texto

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 Professor Carl Schaschke is a chemical engineer hav-
ing worked first at BNFL at Sellafield in Cumbria. After 
then completing a PhD, his academic career began 
at Napier University. He is currently professor at the 
University of Strathclyde. He has had several second-
ments including to the Fawley oil refinery and BBC 
TV’s Tomorrow’s World . His teaching and research 
interests include chemical engineering applications 
under extreme conditions. He is a Fellow of IChemE 
and UK representative of the EFCE Working Party on 
High Pressure Technology. He is married with two 
daughters, Emily and Rebecca.
 A
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of an entry go to the dictionary’s web page at www.
oup.com/uk/reference/resources/chemeng , click 
on Web links in the Resources section and locate 
the entry in the alphabetical list, then click straight 
through to the relevant websites.
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 A Dictionary of 
Chemical 
Engineering
 
 1
 CARL SCHASCHKE 
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 1 
Great Clarendon Street, Oxford, OX2 6DP, 
United Kingdom 
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The moral rights of the author have been asserted 
First edition published in 2014 
Impression: 1 
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referenced in this work. 
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 Contents 
 Preface vii 
 Dictionary 1
 Appendices
 1. SI prefixes and multiplication factors 422 
 2. Derived units 423 
 3. Derived units in SI and c.g.s. 424 
 4. Abbreviations used for piping and instrumentation diagrams (P&IDs) 425 
 5. Dimensions and units 426 
 6. Greek alphabet 427 
 7. Periodic table 428 
 8. Fundamental constants 429 
 9. Recommended web links 430 
 Bibliography 433 
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 Preface
 The purpose of this dictionary is to provide a quick, useful, and comprehensive reference 
to commonly used and, in some case, less commonly used terms from the field of chemical 
 engineering. As with any dictionary, it is intended to provide definitions to words; it is not 
merely a brief glossary of terms, nor is it intended to be encyclopedic, with lengthy and overly 
long explanations. It is aimed at students at school and undergraduate students who will en-
counter, perhaps for the first time, unfamiliar technical terms. It is also aimed at postgraduates 
engaged in chemical engineering research as well as practitioners of chemical engineering in 
industry who may require clarification regarding terms. This dictionary is also aimed at the 
general reader who in the course of their work or daily lives may encounter unfamiliar terms.
 The focus of the dictionary is scientific and engineering terms. It includes core and funda-
mental terms commonly encountered across all degree programmes of chemical engineering 
worldwide. It includes many scientific and engineering concepts, laws, theories, and hypoth-
eses. It includes significant organizations, international legislation, and biographical notes of 
influential scientists and engineers who have contributed to the development of the discipline. 
There are definitions of many types of specialist process equipment encountered in chemical 
engineering. This dictionary should therefore enable the reader to distinguish between a lute 
and a dead leg or a Hortonsphere and a holley-mott. Being a diverse discipline, there is an em-
phasis on established processes across a wide range of industries spanning nuclear, mineral, 
oil and gas, food, and pharmaceutical processing. Some older or former processes are also 
included where their usage was pioneering at the time or influenced later processes. Products, 
raw materials, and feedstocks are included, though to a far lesser extent; only those upon which 
major industries are based, such as crude oil, natural gas, minerals, and ores, are included. The 
full details of chemicals and their properties are included in the sister dictionaries such as the 
 Oxford Dictionaryof Chemistry .
 As a branch of engineering in its own right, the roots of chemical engineering extend back 
to the nineteenth century. While many of the original and familiar terms are still in use today 
(such as unit operations attributed to Arthur D. Little), chemical engineering in the twenty-first 
century has expanded considerably and diversified into many new technological fields such 
as renewable energies, nanotechnology, and biomolecular engineering. Many students and 
professional engineers alike encounter new terms almost daily with which they may not be 
familiar or entirely clear. This dictionary therefore aims to provide up-to-date, clear, concise 
terms and definitions, and other useful and valuable information that can be used as a quick 
reference source.
 The dictionary features over 3,000 of the most commonly encountered terms, although the 
number actually used by chemical engineers is far greater! There are many cases where words 
are used uniquely within a particular industry, or within a single industrial organization, and 
are not be found anywhere else. These have not been included. In providing a definition of 
each of the included words, the aim has been to be inclusive of all aspects of chemical en-
gineering without being too general. If one starts with the very name chemical engineering , 
there are no doubt as many definitions as chemical engineers! Founding member of the In-
stitution of Chemical Engineers Norman Swindin once described chemical engineering as 
 engineering without wheels . An amusing definition but it falls a long way short of being helpful 
or informative.
 The SI system of units has been used throughout although it is recognized that British Imper-
ial and American customary units are still widely used in many industries. Reference has been 
made to commonly encountered units and conversions presented where appropriate.
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Preface viii
 In the preparation of this dictionary, I am indebted to many people who have assisted in 
suggesting words, their comments and corrections. Any errors, omissions, misprints, or ob-
scurities are entirely my own. My thanks to the editorial staff of Oxford University Press and in 
particular Judith Wilson, Jamie Crowther, and Clare Jones, as well as thanks to the copy- editor, 
Marilyn Inglis, and the proofreader, Sarah Chatwin , for their attentive and invaluable work. 
Finally, this book could not have been produced without the support of my wife Melodie and 
my daughters Emily and Rebecca.
 Carl Schaschke 
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A
 ABE fermentation Another name for the * Weizmann process used for the produc-
tion of acetone, butanol, and ethanol using the acid-resistant bacterium Clostridium 
acetobutylicum. 
 ablation The removal of material by * erosion , * evaporation , or * chemical reaction . For 
short-term protection against high temperatures as a form of fire protection or fireproofing 
of process equipment, sacrificial materials are used such that during a fire there is resist-
ance and protection to the equipment beneath for a sufficient period of time.
 ablimaton See sublimation .
 abscissa The horizontal or x-coordinate in a two-dimensional Cartesian coordinate 
 system such as a chart or graph. The * ordinate is the vertical or y-coordinate.
 absolute Denoting a number or a measurement that does not rely on a standard refer-
ence value.
 absolute density The mass per unit volume of a substance. It is the density of the actual 
substance and does not include any free space that may be between particles. The SI units 
are kg m –3 .
 absolute error The difference between a measured value and its true value.
 absolute filter A type of filter used to remove all particles that may be present in the 
flow of gas into or out of a process. Absolute filters are used for ensuring the sterile flow of 
air or oxygen to biological reactors as well as for clean rooms and sterile cabinets used for 
analytical work. Unlike an * air filter , the pore sizes are smaller than the expected particle 
size. With a typical uniform pore size of 0.2 μm, the pressure drop is greater than that of air 
filters made from fibrous materials.
 absolute humidity The amount of water in air expressed as the mass of water vapour 
per unit mass of dry air for a particular temperature and pressure condition. The SI units 
are kg 
water
 kg 
air
 –1 .
 absolute pressure The measurement of gas or air pressure relative to the pressure in a 
total vacuum. In comparison, the * gauge pressure is measured above atmospheric pres-
sure, which is variable.
 absolute roughness (Symbol ε) The roughness of a solid surface expressed as the aver-
age height of undulations and imperfections. It is measured using an instrument that draws 
a stylus over the surface. The roughness of the inner surface of a pipe wall used to transport 
fluids with turbulent flow has the effect of increasing frictional pressure drop. Expressed 
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absolute temperature 2
as a ratio with internal pipe diameter, it is used in determining the friction factor of fluids 
flowing in pipes with turbulent flow. See relative roughness .
 absolute temperature See kelvin .
 absolute viscosity See viscosity .
 absolute zero The lowest possible thermal energy state of a material. This corresponds 
to 0 K.
 absorbed dose See dose .
 absorber 1. A material that is capable of stopping ionizing radiation. * Alpha particles 
can be readily stopped by a sheet of paper whereas beta radiation can be resisted by a cen-
timetre of aluminium. Gamma radiation is absorbed by materials with a high density, such 
as steel and concrete. Neutron absorbers include boron, hafnium, and cadmium and are 
used in the control rods in nuclear reactors. 2. A shortened name for an * absorption tower 
or column.
 absorption A mass transfer process in which one or more gases in a gaseous mixture is 
transferred into a liquid solvent or a solid. It is the most common form of separation of low 
molecular weight materials. Absorption is often used to remove gases from gas streams that 
may be harmful downstream or when released from the process. The absorption factor is 
used to determine the ease with which a component will absorb into the liquid phase and 
is based on liquid and vapour flow rates as well as the vapour liquid equilibrium for the 
component. For example, ammonia can be absorbed from a gas stream using water as the 
scrubbing liquid. Compare adsorption .
 absorption tower A tall vertical column containing a packing material in which a gas 
is absorbed by intimate contact with a liquid flowing downwards under the influence of 
gravity. The gas can be admitted either countercurrent or cocurrent to the flow of liquid in 
which one or more of the gaseous components are absorbed into the liquid. The minimum 
flow rate of scrubbing liquid required to achieve an absorption duty requires an infinite 
height of packing. In practice, a higher liquid rate is used to achieve a compromise between 
capital cost (i.e. height of column) and the operating cost (i.e. liquid flow rate). It is also 
known as a * scrubber. 
 absorptivity The portion of radiant thermal energy falling on a surface which is con-
verted to heat with the remainder being either reflected or transmitted. The absorptivity 
is dependent on the wavelength of the energy and the properties of the surface including 
colour. Compare reflectivity ; transmissivity.
 accelerant A substance used to initiate and develop a fire. Flammable liquids are the 
most common form of accelerants.
 acceleration (Symbol a ) The rate of change of speed or velocity with respect to time. If 
the acceleration is constant then the final velocity, v , of a body that is initially moving with 
a velocity u after time t , is v u at= + . If the acceleration is not constant, then the acceleration 
can be found from: 
 
a
dv
dt
d s
dt
= =
2
2
a
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3 Acheson process 
 where s is the distance moved by the body. In the case of motion in a circle, the acceleration 
is v r2 / and directed to the centre of the circle of radius r.
 acceleration due to gravity (Symbol g) The acceleration experienced by a body due 
to the Earth’s gravitational field. The acceleration is normally taken as 9.806 65 m s –2 al-
though it does vary by small amounts over the Earth’s surface and with altitude.
 acceleration phase The rapid growth of the culture of microorganisms in a bioreactor 
prior to the * log phase . After the medium within a bioreactor has been inoculated with a 
small population of microorganisms, there is an initial * lag phase of no growth in which 
they adjust to their new environment. Cell division then occurs at an increasing rate until 
the maximum growth rate is reached. The log or exponential phase corresponds to the 
rapid cell division such that the logarithm of the population increase with time is constant. 
As the substrate eventually becomes exhausted, this is then followed by a deceleration 
phase prior to the * stationary phase .
 accelerator A substance that alters the rate of a chemical reaction such as a * catalyst .
 accumulator A device used to smooth the rate of flow from a reciprocating pump and 
prevent the destructive effects of * water hammer from occurring. It consists of a vessel lo-
cated on the pipe close to the pump with a * non-return valve preventing return flow back 
to the pump. The vessel contains a gas or a bladder bag although some use springs. As the 
pump discharges, some of the fluid enters the accumulator compressing the gas or spring. 
At the point of valve closure, the gas or spring expands allowing the accumulated volume 
to discharge through the pipe.
 accuracy A measure of the closeness or agreement of a numerical value to a true value. It 
is expressed as either * significant figures or decimal places depending on whether propor-
tional or absolute accuracy is important. For example, a number written as 5.425 normally 
assumes that the four figures are meaningful. It would be incorrect to write the number to 
a precision of five significant figures unless the * error in the estimate is indicated such as 
5.4250 ± 0.0005. Compare precision .
 acentric factor A parameter used in * equations of state to estimate physical and thermo-
dynamic properties. It is used to characterize the acentricity of molecules in reduced-state 
correlations along with reduced pressure and reduced temperature.
 acetate process A process for the production of cellulose fibres used for textiles. There 
are two methods: 1. The cellulose is obtained from wood pulp and dissolved in carbon di-
sulphide and sodium hydroxide. The thick brown liquid that contains cellulose xanthate is 
forced through orifices into acid. The xanthate decomposes to leave a cellulose fibre known 
as viscose rayon. 2. The cellulose obtained from wood pulp and cellulose acetate is formed 
by dissolving in acetone. The solution is forced through orifices and the solvent is allowed 
to evaporate leaving a cellulose fibre of acetate rayon.
 ACHEMA (Ausstellungstagung für chemisches Apparatewesen) A triennial trade 
fair for chemical technology and biotechnology held in Frankfurt, Germany.
A
• Ofi cial website of ACHEMA.
 Acheson process A process used for the production of graphite. It involves heating 
coke mixed with clay to a very high temperature. At a temperature in excess of 4,000 o C, 
a
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acid 4
silicon carbide is formed leaving graphite. It is named after the American inventor Edward 
Goodrich Acheson (1856–1931) who patented the process in 1896.
 acid A chemical compound or material containing hydrogen that has the tendency to lose 
protons and form hydrogen ions in solution. Solutions of acids have * pH values less than 7.
 acid egg An egg-shaped vessel used to transport highly corrosive acids. The container 
has inlet and outlet pipes and is filled with a charge of liquid to be transported. Another 
pipe is used to admit compressed air or another gas. The pressure of the gas on the liquid 
surface forces the liquid through the discharge pipe that extends down into the liquid. The 
acid egg is not very efficient as the compressed air or gas is usually blown off when the op-
eration is completed. See montejus .
 acid gas Natural gas, which consists mainly of methane, but also contains significant 
amounts of carbon dioxide, hydrogen sulphide, and other acidic contaminants. Natural 
gas from offshore reservoirs that contain these corrosive and toxic contaminants are re-
quired to be removed or reduced at the platform before export using an * amine gas treating 
process . Compare sour gas .
 acid number A measure of the acidity of oils such as crude oil, mineral oils, and biodies-
els. It is expressed as the mass in milligrams of potassium hydroxide titrated in one gram of 
the oil required to neutralize it.
 acid rain A precipitation of rain that has a pH below that of typical rain, which is around 
pH 5.6. Rainwater is naturally acidic due to the absorption of carbon dioxide from the air to 
form carbonic acid. However, rainwater will also absorb other gases such as sulphur diox-
ide and various oxides of nitrogen that have been released into the atmosphere as pollutant 
gases through processes such as the combustion of fossil fuels and from car exhausts. The 
dissolved gases form sulphuric and nitric acids with pH values of less than 5.0 and have 
an adverse effect on trees and plants. Acid rain causes damage to leaves and increases the 
acidity of the soil preventing further growth. The water run-off into rivers and lakes also 
prevents freshwater fish from thriving, leaving the water sterile, and has a major impact on 
the ecosystem.
 activated carbon A compound of powdered or granular amorphous carbon mainly 
made from coconut shells. It has a very high specific surface area used to adsorb vapours 
and gases. With a surface area typically of around 1,000 m 2 per gram, it is widely used to 
 adsorb vapours and gases. The amount of substance that can be adsorbed is proportional 
to the absolute temperature and pressure. The activated carbon can be reactivated for 
reuse using steam to strip the adsorbents and recover the carbon. Activated carbon is used 
in water and air purification, and used in gas masks for the removal of harmful gases. It is 
also known as activated charcoal and active carbon .
 activated sludge process A process used in the treatment of sewage and wastewater. 
 * Sludge is formed when air is bubbled through the sewage resulting in the aggregation of 
flocs. These contain denitrifying bacteria that are capable of decomposing organic sub-
stances. Aeration ensures a high level of dissolved oxygen and helps to reduce the * biologi-
cal oxygen demand . Stirring of the sludge can also aid the process.
 activation energy (Symbol E 
a
 ) The minimum energy required to activate one mole of a 
substance to cause a * chemical reaction to take place.For a chemical reaction to proceed, 
a
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5 ADC 
the reactants are converted to products in which the energy increases to a maximum and 
falls to the energy of the products ( see Fig. 1 ). The activation energy is the difference be-
tween the maximum energy and the energy of the reactants and is therefore the energy 
than needs to be overcome. See arrhenius equation . 
 active site 1. An available location on the surface of a * catalyst available for reactants 
to bind and result in a * chemical reaction taking place. The active site can be blocked by a 
chemical agent or * poison thereby reducing the effectiveness of the catalyst. 2. On the sur-
face of an enzyme, the active site is the location to which a substrate binds. The binding of 
the substrate to the enzyme is dependent on the conformation or 3-D shape of the protein. 
Inhibition prevents the binding from taking place by altering the conformation thereby 
preventing the substrate from binding, or by blocking the site.
 activity 1. The change in one condition to another expressed as a ratio. Examples include 
 * water activity , and chemical activity. The activity of a chemical reaction is used in place of 
concentration in equilibrium constants for reactions involving non-ideal gases and solu-
tions. 2. A quantitative term used to characterize the number of atomic nuclei that disin-
tegrate in a radioactive substance per unit time. It is measured in * becquerel (Bq) where 
one Bq is equal to one disintegration per second. The unit of activity replaces the former 
unit of the curie (Ci) where one Ci is equal to 37 × 10 10 Bq. The specific activity is the activity 
per unit mass of a pure radioisotope. 3. (Symbol U) The amount of an enzyme present in a 
biologically catalyzed reaction. It is usually expressed in terms of units of activity based on 
the rate of the reaction that the enzyme catalyzes. The international unit of activity is the 
amount of enzyme that will convert one μmol of substrate to a product in one minute under 
defined conditions. These are usually 25 o C and the optimum pH. 4. A thermodynamic pa-
rameter that measures the so-called active concentration of a substance, a , in a chemical 
system, and is in contrast to the molecular concentration, c . It is related a fc= where f is a 
dimensionless parameter and approaches unity in dilute solutions.
 activity coefficient (Symbol γ) A correction factor that allows for the deviation from 
ideal behaviour of a gas or solution.
 ADC See analogue-to-digital converter .
reactants
time
products
activation energy
e
n
e
rg
y
 Fig. 1 
a
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additive 6
 additive A substance added in a small amount to another or mixture to improve the per-
formance or properties in some way. Additives are added to polymers to enhance their 
stabilizing properties. Additives are added to foods as preservatives, and to enhance col-
our and flavour. Additives can also provide corrosion resistance, alter surface tension, and 
viscosity etc.
 adiabatic A thermodynamic process that takes place without heat transfer to or from 
an external source. When a fluid is compressed adiabatically, there is an increase in tem-
perature of the fluid. Likewise, adiabatic cooling occurs when the pressure of the fluid 
is reduced without any heat exchange to the surrounding. Adiabatic expansion of a 
fluid occurs without any heat transfer with the surroundings. Adiabatic compression is 
the compression of a gas without any transfer of heat to the surroundings. It results in an 
 increase in the temperature of the gas undergoing compression.
 adiabatic efficiency 1. The ratio of the work required for adiabatic compression to the 
real work input. 2. The ratio of kinetic energy of a fluid through a valve to the kinetic energy 
obtained through the process of * adiabatic expansion.
 adiabatic flame temperature The theoretical temperature of a flame during the 
combustion of a fuel in oxygen considered when there is no loss of energy. The tempera-
ture is dependent on whether the combustion process occurs at either constant pressure 
or constant volume. At constant pressure, the adiabatic flame temperature is due to the 
complete combustion of the fuel with no heat transfer or changes in kinetic or potential 
energy. Constant volume combustion results in a lower flame temperature since some of 
the energy is otherwise used as work to change the pressure.
 adiabatic flash Another name for * flash evaporation , which involves the rapid isenthal-
pic evaporation of a saturated liquid into a liquid and vapour by the reduction in pressure.
 adiabatic process A physical or chemical process without the loss or gain of heat. The 
 adiabatic equation pV kγ = describes the relationship between the pressure of an * ideal 
gas and its volume where γ is the ratio of the specific heat capacities of the gas and k is a 
constant.
 adiabatic saturation temperature The equilibrium temperature attained when a 
liquid and gas are brought into contact with no work or heat transfer done.
 adjutage A tube inserted into a vessel to obtain a measure of its pressure or to allow the 
discharge of its contents.
 adsorbate A substance that is adsorbed from a gas or liquid onto a solid surface or 
 adsorbent during an * adsorption process.
 adsorption A process in which components in gases, liquids, or dissolved substances 
are selectively held on the surface of a solid. It is used to remove components that may 
otherwise be harmful if released into the environment or may cause process difficulties 
further downstream such as causing the poisoning of a catalyst. Adsorption usually takes 
place in * fixed beds .
 adsorption isotherm The relationship between the mass of * adsorbate taken up per 
unit mass of adsorbent at constant pressure, if a gas, or at constant temperature, if in a 
solution. The * BET , * Langmuir , * Freundlich , and * Temkin adsorption isotherm equations 
a
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7 agitation intensity
are empirical equations used to describe the surface available for adsorption at constant 
pressure for gases and constant temperature for solutions.
 advection The natural movement of a fluid such as air resulting in horizontal motion 
caused by local pressure differences. It differs from * convection since it does not include 
the effects of diffusion.
 aeration The introduction and movement of air or oxygen at a low flow rate through a liq-
uid medium such as a * bioreactor or * activated sludge process . Aeration is used to provide 
oxygen to microorganisms that are responsible for biologically catalyzed reactions. The 
oxygen is usually introduced through a * sparger as small bubbles that have a high surface 
area. Aeration is used to promote effective mass transfer of the oxygen to the liquid medium 
and therefore microorganisms.
 aeration number A dimensionless number, N 
a 
, used in the aeration-mixing of bioreac-
tors and relates the gas flow rate, G , to the impeller speed, N , and diameter, D , as:
 N
G
ND
a = 3 
 aerobic process A biochemical process involving microorganisms that require the pres-
ence of oxygen, usually in the form of air. Many organisms require the presence of oxygen to 
survive and grow, such as plants, animals, and many microorganisms. They are dependent 
on oxygen for the breakdown of sugars into carbon dioxide and water, and for the releaseof 
energy through aerobic respiration. In comparison, anaerobic respiration releases energy 
in the absence of oxygen.
 aerogel A highly porous material based on metal oxides or silica. It has a very low density 
below 10 kg m –3 and has excellent heat and electrical resistance as well as acoustic proper-
ties. Aerogels can be formed using the process of * supercritical drying using carbon dioxide 
to remove a solvent such as ethanol used in their formation. Being supercritical and with-
out a gas–liquid interface, it avoids the crushing effects of capillary forces on the porous 
structure during a conventional drying process.
 aerosol A dispersion of fine droplets of liquid or particles of solid within a gas such as 
air. The particles are often very small and colloidal in size. An aerosol spray can contains 
propellants that are liquefied under pressure and used to create an aerosol when released 
into the air.
 agglomeration The process of bringing a suspension of small or fine particles together 
to form larger and more coarse particles or aggregates.
 aggregated fluidization See fluidization .
 aggregation The formation of large groups of molecules or particles. With particles, ag-
gregation consists of both * flocculation and * coagulation .
 agitated vessel A vessel in which the contents are stirred by mechanical means through 
the use of an agitator, paddle, or stirrer. Impellers and propellers are commonly used to 
provide good mixing characteristics. It is also known as a stirred tank. 
 agitation intensity A measure of the power consumption of the shaft of an agitator 
used to mix a liquid in a stirred tank or * agitated vessel . Agitation intensities are expressed 
a
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agitator 8
as the power supplied per unit volume of liquid. The SI units are W m –3 . The magnitude of 
the agitation intensity is dependent on the nature of the liquid being stirred. Biological so-
lutions containing flocculating materials are significantly affected by the level of agitation.
 agitator A simple stirring device used to provide turbulence and mixing of the contents 
of a vessel containing a liquid. It is typically used to provide homogeneity, provide good 
oxygen transfer in fermentation vessels, and in the prevention of particles settling. 
An agitator consists of blades attached to a rotating shaft. Impellers have flat blades and 
provide radial flow patterns whereas propellers provide axial flow movement. Paddle agita-
tors consist of tilted flat blades providing a combination of radial and axial flow movement. 
Selection of the appropriate agitator depends on the processing requirements, the fluid 
properties, and the materials of construction.
 AIChE See American Institute of Chemical Engineers .
 air An odourless and colourless mixture of gases and vapours that surround the Earth. 
At sea level, the composition of dry air is mainly nitrogen (78.09 %) and oxygen (20.95 %), 
with an average relative molecular weight of 29. Other gases include argon (0.93 %), carbon 
dioxide (0.03 %), neon (1.8 × 10 –3 %), helium (5.2 × 10 –4 %), and lesser amounts of methane, 
krypton, hydrogen, nitrous oxide, xenon, and radon in decreasing amounts, respectively. 
Air is a common source of oxygen used in many processes such as * combustion .
 air conditioning The process of controlling the environmental air conditions in build-
ings through control of the temperature and level of relative humidity, as well as through 
filtration of particles to provide human comfort. The movement and cleanliness of the air 
are also involved.
 air filter A type of filter used to remove particles such as dust, soot, and microorganisms 
from the flow of air. They are often used for ensuring a sterile flow of air or oxygen to bioreac-
tors as well as for clean rooms and sterile cabinets used for analytical work. The pore sizes of 
the filter are larger than the particle size to be removed such that the filter relies on the depth 
of the filter to entrap the particles within a fibrous mesh structure. Fibrous filters are relatively 
cheap and robust, and have a low pressure drop in comparison with * absolute filters .
 air-lift A pumping device used to raise a liquid from a depth such as a well. It consists of 
a vertical pipe extending down into the well into which compressed air is injected at the 
 bottom. As the air bubbles rise, the reduced hydrostatic pressure results in a flow of liquid 
up the leg. The air or gas is disengaged from the liquid at the top of the leg. It is used for 
raising oil from wells.
 air-lift reactor A type of bubble column reactor into which air is sparged at the bottom 
as bubbles to promote oxygen transfer and cause circulation of the liquid. The reactor is 
cylindrical and mounted on its axis. It has an inner tube up which the air or oxygen rises. 
An external-loop air-lift-type reactor consists of a U-tube within which the sparging takes 
place promoting oxygen transfer and liquid circulation.
 air lock 1. Trapped air or some other gas or vapour within a pipe that prevents the inten-
tional flow of a liquid. 2. The intentional seal in a process that relies on a differential pres-
sure to prevent the undesirable loss of material.
 air pollution The release of particles, vapours, and gases into the environment that are 
harmful to human health and to the environment such as plants, forests, and animals. Carbon 
dioxide is a product from the combustion of fossil fuels in power stations, vehicles, aeroplanes, 
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9 ALARP 
and numerous industrial processes, and is a greenhouse gas responsible for contributing to 
the warming of the Earth’s atmosphere. Methane is another greenhouse gas as are chlorofluor-
ocarbons (CFCs), which were once widely used as refrigerants and as aerosol propellants but 
are now banned due to their known damaging effect on the Earth’s ozone layer. Sulphur diox-
ide is another product of the combustion of fossil fuels and is known as the cause of * acid rain .
 In the UK, an Act of Parliament was introduced in 1956 to reduce the level of air pollution. 
It was a landmark in environmental protection and was responsible for reducing the level 
of smoke pollution as well as sulphur dioxide emitted into the environment.
 In the US, the Clean Air Act introduced in 1963, together with its subsequent amend-
ments as a federal law, has been responsible for controlling air pollution. Other govern-
ments have also taken measures to control air pollution and limit the emission of carbon 
dioxide and other greenhouse gases. The Kyoto Protocol is an international agreement 
between countries to reduce the emissions of carbon dioxide emissions and restrict or 
ban the emission of certain chemicals such as CFCs. One way of restricting carbon dioxide 
emissions is to raise the level of taxation on fuels so that people and industrial companies 
have greater incentives to conserve energy and pollute less.
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• Ofi cial website of Environmental Protection UK.
 air separator A device used to separate solid or liquid particles from air in which 
 centrifugal force is used. The device has a cylindrical body with a conical base. The particle-
containing air enters tangentially and the particles leave from the bottom while particle-
free air leaves from the top. It is also known as a * cyclone separator .
 air-to-close A type of pneumatically operated control valve that automatically opens in 
the event of a loss of instrument air pressure. An air-to-open valve is a pneumatically oper-
ated control valve that automatically closes in the event of aloss of instrument air pressure. 
For example, the fuel supply to a furnace should automatically shut on air failure.
 ALARA An abbreviation for a s l ow a s r easonably a chievable, it is a management tool used 
in the controlling of risks. For example, it is used to manage the exposure to chemicals and 
ionizing radiation doses in humans working in the nuclear industry. Compare alarp .
 alarm An indicator used to alert operators and personnel that there has been a significant 
deviation from an expected measured * process variable or process condition. The alarm 
may be audible in the form of a siren, bell, or other noise, or may be a flashing or continuous 
light signal. Alarms are a feature of control panels where the process is displayed on screens 
with associated alarms. Alarm flooding is a condition in which alarms appear on control 
panels in * control rooms at a rate which exceeds that which an operator can comprehend 
or respond to quickly or effectively. It therefore prevents the operator from identifying the 
cause of the process upset and consequently limits the scope for an effective response.
 ALARP An abbreviation for a s l ow a s r easonably p racticable, it is a management tool used 
to determine the level to which risks are to be assessed and controlled. It involves a rigor-
ous and systematic assessment of the minimization of risk and the costs in terms of time, 
money, and effort to achieve it. As a form of good practice requiring judgement between 
risk and societal benefit, it was developed through the UK parliamentary Health and Safety 
at Work Act (1974). Outside the UK, similar forms of engineering practice are used and 
this includes * ALARA (as low as reasonably achievable) in the US for radiation protection.
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• Ofi cial website of the Health and Safety Executive UK offering risk assessment advice.
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algorithm 10
 algorithm A mathematical method or operation that follows a scheme of calculations or 
steps designed to be repeated such that the result from one calculation forms the basis of 
the next. The stage-by-stage computation of the liquid and vapour flows and compositions 
in a distillation process is based on a defined algorithm.
 aliquot A portion of a total amount of something. For example, a prepared solution of 
reactants may be fed to a process in aliquots.
 alkali A metal hydroxide that produces hydroxyl (OH-) ions in solution.
 alkane A saturated aliphatic hydrocarbon that has the general formula C 
n
 H 
2n+2
 . Forming 
a homologous series, the smallest is methane (CH 
4
 ), followed by ethane (C 
2
 H 
6
 ), propane 
(C 
3
 H 
8
 ), butane (C 
4
 H 
10
 ), etc. The smaller alkanes are gases at ambient temperature. Meth-
ane is found in oil and gas reservoirs, and in lesser amounts in coal seams. Methane is also 
the product of the decay of organic material by bacteria and produced from * anaerobic 
digestion processes. Mixtures of short-chain alkane gases can be separated by distillation, 
either by condensing the liquids using low temperature or by pressurization, or a combina-
tion of the two. They were formerly known as paraffins although this name is still used in 
certain industries such as petroleum refining.
 alkene An unsaturated aliphatic hydrocarbon that has the general formula C 
n
 H 
2n
 . They 
comprise one or more carbon–carbon double bonds. They were formerly known as ole-
fins . The series starts with ethene (ethylene) with the formula C 
2
 H 
4
 , followed by propene 
(propylene) C 
3
 H 
6
 , butene (butylene) C 
4
 H 
8
 , etc. Isomerism occurs with the higher alkenes 
beginning with butane for which there are two isomers: but-1-ene and but-2-ene that differ 
by the position of the double bond. Alkenes can undergo * polymerization to form thermo-
plastics such as polyethene (polyethylene).
 alkylation A process in which an alkyl group is added to another organic molecule 
such as by removing a hydrogen atom from an * alkane and adding a methyl group. In the 
refining of * crude oil , it is used to upgrade petroleum through the alkylation of isobutane 
with * alkenes (olefins) such as propene, in the presence of either sulphuric or hydro-
fluoric acid as a * catalyst . The reaction takes place as a two-phase reaction at ambient 
temperature. The reaction products are a mixture of branched hydrocarbons with a high 
 * octane rating . The octane number of the mixture depends mainly on the kind of alkenes 
used. Iso-octane has an octane rating of 100 and is the result of reacting isobutane with 
butene (butylene).
 alkyne An unsaturated aliphatic hydrocarbon that has the general formula C 
n
 H 
2n-2
 . 
These are characterized by a triple carbon–carbon bond. Alkynes that feature a single 
triple bond form a homologous series beginning with ethyne (acetylene) C 
2
 H 
2
 , followed 
by propyne (propylene) C 
3
 H 
5
 , butyne (butylene) C 
4
 H 
6
 , etc. They were formerly known as 
acetylenes.
 allotropy The existence of different forms of the same element in the same phase, known 
as allotropes . Carbon has the allotropes of graphite, diamond, graphene, and fullerenes. 
Many other elements exhibit allotropy.
 alloy A material that consists of two or more metals, or a metal and a non-metal such as 
carbon. Pewter is made from tin with lesser amounts of lead along with small amounts 
of antimony and copper. Steel is made from iron alloys and contains a small amount of 
carbon.
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11 American Petroleum Institute
 alpha particle A positively charged particle emitted by various radioactive materi-
als such as uranium during radioactive decay. The particle consists of two neutrons and 
two protons, and is therefore identical to the nucleus of a helium atom. The result of this 
 * radioactive decay is that the original element is gradually converted into another element 
with a decreased atomic number and mass. Alpha particle emissions, or alpha decay may 
occur at the same time as * beta decay . It can be stopped by a sheet of paper and is harmful 
to humans only if the substance emitting the alpha particles is ingested, inhaled, or enters 
the body through wounds.
 alternator Another name for an electromagnetic * generator used to produce alternating 
current in a power station.
 Amagat’s law A law that states that for an ideal gas, the total volume occupied by a gase-
ous mixture is equal to the sum of the pure component volumes:
 V V V VA B C= + + +  . . .
 It is named after French physicist Emile Hilaire Amagat (1841–1915).
 amalgam 1. An alloy of mercury with another metal, such as silver, used in dentistry. 
Most metals form an amalgam with mercury with the exception of iron and platinum. 
 2. A white mineral consisting of mercury and silver that occurs in deposits of silver and cin-
nabar, which is a bright-red mineral form of mercuric chloride found near areas of volcanic 
activity and hot springs.
 ambient temperature The temperature of the surrounding atmospheric air. Ambient 
air temperature can affect the operation of process equipment, instruments, and control. It 
is sometimes referred to as room temperature.
 American Institute of Chemical Engineers (AIChE) A professional society based 
in the US with a membership of over 43,000 chemical engineers in a hundred countries. 
Founded in 1908, AIChE was established to provide its members with a focal point to share 
ideas and grow the discipline. Today it provides its members with technical resources and 
organizes major conferences, as well assetting accreditation standards for chemical engin-
eering education, and setting guidelines for government agencies.
A
• Ofi cial website of the American Institute of Chemical Engineers.
 American National Standards Institute (ANSI) An American not-for-profit organ-
ization responsible for the accreditation of organizations that write industrial standards. It 
was founded in 1918.
A
• Ofi cial website of the American National Standards Institute.
 American Petroleum Institute (API) An American professional trade organization 
that represents all aspects of the US oil and natural gas industry. It was formed after the 
First World War (1914–18) as a consortium of oil and gas companies to help the recovery 
from the war by working together. It was formally established in 1919 as a means of co-
operation with the government in all matters of national concern and to develop the wider 
interests of the petroleum industry. It sets standards and recommends practices, covering 
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American Society for Testing Materials 12
all aspects of the industry, and promoting the use of safe, proven, and sound engineering 
practices.
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• Ofi cial website of the American Petroleum Institute.
 American Society for Testing Materials (ASTM International) An international 
standards organization that develops and publishes voluntary technical standards for ma-
terials, products, systems, and services. Unlike ANSI, it is not a national standards body. It 
has been responsible for developing and maintaining more than 12,000 standards and the 
 Annual Book of ASTM Standards consists of 77 volumes.
A
• Ofi cial website of ASTM International.
 American Society of Mechanical Engineers (ASME) A professional organization 
based in the US that provides its members with technical resources focusing on technical, 
educational, and research matters. It also produces standards such as ASME VIII, which is 
an accepted code for the design of pressure vessels and heat exchangers covering design, 
material selection, fabrication, inspection, and testing.
A
• Ofi cial website of the American Society of Mechanical Engineers.
 AMIChemE Post-nominal letters used after a person’s name to indicate that they are an 
Associate Member of the * Institution of Chemical Engineers .
 amine gas treating process A post-combustion process used to remove * acid gases 
such as carbon dioxide, hydrogen sulphide, and mercaptans from natural gas using an 
amine chemical solvent to react and form reversible compounds. Carbon dioxide is re-
quired to be removed since it reduces the calorific value of natural gas and forms carbonic 
acid in water which is corrosive as well as having a * global warming potential . The process 
involves the reversible reaction of the gas with an amine to form an amine salt. Various 
amines are used including monoethanolamine. The amine solution is sprayed into a large 
tower and absorbs the hydrogen sulphide as well as carbon dioxide from upflowing gases. 
A regenerator operating at a higher temperature is used to strip the amine solution of the 
gases for reuse. See gas sweetening .
 ammonia-soda process See solvay process .
 amorphous A non-crystalline solid form of matter in which the atoms or molecules are 
arranged at random within a three-dimensional structure. Glass is an example of an amor-
phous solid. Compare crystal .
 amount of substance (Symbol n) A measure of the number of entities present in a 
substance, such as atoms, molecules, ions, and electrons, etc., expressed in moles. For ex-
ample, the amount of an element is proportional to the number of atoms present where 
one mole of that element is equal to 6.022 1367 × 10 23 atoms, which is * Avogadro’s constant . 
It is given by: 
 n
N
N A
=
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13 analysis 
 where N is the number of atoms and N 
A
 is Avogadro’s constant. The SI unit is the mole. It is 
also known as chemical amount .
 ampere (Symbol A) The SI unit of electric current, it is the constant flow of current that is 
maintained between two parallel conductors of infinite length and of negligible cross sec-
tion that produces a force of 2 × 10 –7 newtons per metre (Nm –1 ) between them. It is named 
after the French physicist and mathematician André Ampère (1775–1836), who made sig-
nificant discoveries in electricity and magnetism.
 ampere-hour A practical unit of electric charge as the quantity that flows in one hour 
through a conductor carrying a current of one ampere. It is equivalent to 3,600 coulombs.
 amplitude The maximum value of varying quantity from its mean or base value. For 
 example, in simple harmonic motion the amplitude of a wave is half the maximum peak-
to-peak value.
 a.m.u. See atomic mass unit .
 anaerobic digester A type of bioreactor used for the * anaerobic digestion of organic 
waste liquids from domestic and industrial sources. The biological process involves the 
use of bacteria in the near absence of oxygen to produce a mixture of methane and carbon 
dioxide, known as * biogas . * Continuous stirred-tank reactors are used for the treatment 
of industrial waste with a continuous inflow and outflow. * Batch processes are used for 
smaller domestic, community, or farm-scale processes.
 anaerobic digestion A biochemical process in which bacteria break down organic 
matter in the absence of oxygen into a mixture of carbon dioxide and methane known as 
 * biogas . The main stages involve hydrolysis, acidogenesis, acetogenesis, and methanogen-
esis. An * anaerobic digester can be operated at a steady-state condition through control of 
temperature for psycrophilic, mesophilic, and thermophilic bacteria, pH, the carbon-to-
nitrogen ratio, organic dry matter content, hydraulic retention time, degree of mixing, the 
availability of nutrients and trace elements, and rate of biogas removal.
 analar reagent A high-purity chemical reagent used for chemical analyses with a de-
fined level of purity.
 analogue signal An electrical signal used in the control of processes as a current or a 
voltage representing temperature, pressure, level, etc. The commonly used electrical cur-
rent signal has a range of 4–20 mA. The voltage range commonly used is 0–5 volts DC.
 analogue-to-digital converter (ADC) Electronic hardware used in the control of pro-
cesses that converts analogue signals such as electrical voltage, current, temperature, and 
pressure into digital data that a computer can process.
 analogy A form of general agreement or similarity between problems, reasoning, meth-
ods, or logic. It is used to compare the results from one particular problem to those of an-
other from a known similarity between them.
 analysis The detailed examination of something such as a mathematical problem using 
the theories of calculus, a chemical substance into its constituent parts, the study of a 
 physical process and its function or operation, the economics of a chemical process or 
business, etc.
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analysis of variance 14
 analysis of variance (ANOVA) One of a number of statistical techniques used to re-
solve and observe the variance between sets of statistical data into components. These 
techniques are used to determine whether the difference between samples is explicable 
as random sampling variation from within the same statistical populations. ANOVA tech-
niques are used in * quality control .
 analyte A substance that is being determined in an analytical procedure.analytical reagent A chemical compound of a known and high purity used in a chem-
ical * analysis .
 ancillary equipment Mechanical equipment used to support or assist a primary item 
of equipment in meeting its functional duties. Pumps, blowers, and heating equipment are 
all ancillary items of equipment used to support main process plant items.
 Andrews, Thomas ( 1813–85 ) An Irish scientist noted for his work on gases. He studied 
chemistry at the University of Glasgow before undertaking further studies in Paris. He then 
attended Trinity College, Dublin before completing his medical studies in Edinburgh and 
then returning to Belfast to set up practice as a physician. When Queen’s College opened 
in 1845, he was appointed professor of chemistry, and also the first vice president of the 
college. During this time, he carried out his most important studies on gases. His three 
main areas of work concerned thermochemistry, the nature of ozone, and the continuity 
of liquid and gaseous states of matter. He was offered a knighthood but declined on the 
grounds of ill health.
 Andrussov, Leonid ( 1896–1988 ) A chemical engineer born in Riga who is noted for de-
veloping a process for the production of hydrogen cyanide based on the oxidation of am-
monia and methane over a platinum catalyst.
 Andrussov process A catalytic process used for the production of hydrogen cyanide 
by the reaction of ammonia, methane, and air at a temperature of around 1,000°C using a 
platinum catalyst: 
 2 3 2 2 64 2 23CH O NH HCN H O+ + → +
 The ammonia in the product gases is removed by gas absorption with sulphuric acid and 
the hydrogen cyanide is absorbed in water. The hydrogen cyanide is used as the prelim-
inary product for the synthesis of polyamide 66 or nylon, and for polymethyl methacrylate. 
It is also called Andrussov oxidation after the inventor who patented the process in 1930s.
 anemometer An instrument used to measure the speed of a gas such as air. It comprises 
cups or vanes that rotate freely and are linked to a tachometer. Hot-wire anemometers 
 feature a heated wire over which the gas or air passes. Since the electrical resistance of 
certain metals such as tungsten is dependent on temperature, the cooling effect of the gas 
over the wire changes its resistance from which the velocity is inferred.
 aneroid An instrument used to measure barometric or atmospheric pressure. It has metal 
bellows as a sensing device.
 angel’s share An amount of Scotch whisky lost by evaporation during the process of 
maturation in wooden casks. Scotch * whisky is stored for a minimum of three years over 
which time the level of whisky can drop by as much 2 per cent per year.
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15 ANOVA 
 angström (Symbol Å) A unit of length equal to 10 –10 m. It is used to measure the 
wavelengths of electromagnetic radiations and was formerly used for the measure of 
intermolecular distances. It has now been replaced by the nanometre (10 –9 m). It is 
named after the Swedish astronomer and professor of physics Anders Jonas Angström 
(1814–74).
 angular momentum (moment of momentum) A measure of the momentum of a 
body caused by its circular motion around an axis of rotation. It is the vector product of 
the position vector and the tangential component of velocity of an object moving about 
a centre of rotation. The angular momentum of a mass m of fluid is mv rθ where v θ is the 
tangential velocity.
 angular velocity (Symbol ω) The rate of change of angular displacement with time: 
 ω
θ
=
d
dt
 The rotational speed of shafts for mixers, centrifugal separators, and centrifugal pump im-
pellers are sometimes expressed in radians per second.
 annealing A heat treatment process used to relieve internal stresses in ferrous and non-
ferrous metals. It involves heating the metal to a specified temperature over a specified 
period of time to soften it. It is then allowed to cool slowly. The annealed metal is less brittle 
with reduced internal stress and is therefore easier to work or machine. A similar process 
is applied to glass.
 annular flow A two-phase flow regime of a gas and a liquid in a vertical pipe or tube 
characterized by a continuous gas core with a wall film of liquid. The flow regime occurs at 
high gas velocities compared with the liquid. There is often a simultaneous flow of the liq-
uid phase entrained in the gas as a fine dispersion of droplets. In horizontal pipes, the effect 
of gravity causes the film to become thicker on the bottom of the pipe. As the gas velocity 
is increased, the film becomes more uniform around the circumference. See multiphase .
 annulus The region between two concentric circles. The area of an annulus is equal to 
π( )/d d1
2
2
2 4− where d 1 and d 2 are the outer and inner radii. An annular gap is the clear-
ance between two concentric pipes or tubes. The use of concentric pipes or tubes is found 
in the nuclear industry as a form of double containment. The central pipe is used to carry 
a radioactive liquid such as plutonium nitrate and the gap in the annular gap is kept under 
reduced pressure. In the event of leaks, the radioactive liquid is retained within the annular 
gap and recovered without release into the environment.
 anode A positive electrode in an electrolytic cell. In the process of electrolysis in which 
electricity is passed through an electrolyte, the electrode attracts electrons from an external 
circuit. Compare cathode .
 anodize An electrolytic process used to provide a hard, smooth, and corrosion-resistant 
surface to aluminium and some other metals. The piece for coating is connected to the 
anode of a DC circuit and is immersed in an acid solution. The flow of current liberates 
oxygen at the surface which reacts with the aluminium to form aluminium oxide. Chromic, 
oxalic, and sulphuric acids are commonly used. The anodized surface may typically have a 
thickness of between 0.005 mm and 0.018 mm.
 ANOVA See analysis of variance .
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anoxic reactor 16
 anoxic reactor A type of anaerobic bioreactor in which oxygen is excluded from the 
cultured bacteria. Anoxia is the absence of molecular oxygen in living tissue cells used to 
indicate the reduction of the oxygen content of the blood below physiological levels.
 antilogarithm (antilog) The inverse function of a * logarithm . That is, a number whose 
logarithm to a given base is a given number. For example, the antilogarithm of 2 to the base 
10 is 100. In natural logarithms, the antilogarithm of x is e x .
 antithixotropic fluids Shear thickening fluids that thicken with time. The viscosity 
of such fluids increases when a shear stress is applied, as in stirring, and is also depend-
ent on the time that the shear stress has been applied. Compare thixotropic fluids . See 
 rheopexy .
 Antoine equation An empirical equation used to determine the vapour pressure of a 
substance as a function of temperature: 
 
log10 p A
B
T C
= −
+
 where p is the vapour pressure, T is the temperature and A, B, and C are empirically deter-
mined constants. The pressure is given in mmHg. It is named after C. Antoine who pub-
lished the equation in 1888.
 Antonov’s rule An empirical equation used to describe the surface tension between two 
liquids in equilibrium being equal to the difference between the surface tension of the two 
liquids when exposed to air.
 APCChE (Asian Pacific Confederation of Chemical Engineering) Founded in 1975, 
it is a not-for-profitorganization that brings together various societies, associations, and 
institutions of chemical engineering in the Asia Pacific region. This covers the thirteen 
countries of China, Korea, Japan, New Zealand, Thailand, India, Philippines, Indonesia, 
Singapore, Australia, Malaysia, Taiwan, and Hong Kong. The American Institute of Chemi-
cal Engineers and the Institution of Chemical Engineers are corresponding members.
 API gravity A measure of the density of petroleum oils used in the US and related to 
 * specific gravity : 
 
O API
SG
= −
141 5
131 5
.
.
 The specific gravity and API gravity refer to the weight per unit volume at 15.6 o C (60 o F). 
Most crude oils range between 20 and 45 o API.
 apparent density The mass per unit volume of a material that includes voids. It is a 
measure of the bulk of the material. Compare specific density .
 apparent viscosity (Symbol η) The viscosity of a fluid as a measure of the ratio of the 
shear stress to shear rate and used for non-Newtonian fluids such as drilling muds.
 approximation A mathematical process used to describe roughly the value of a quantity 
of something that is not exact but is sufficiently close to a known or correct value within 
acceptable boundaries of error.
 aqueous Used to denote solutions in which water is the solvent.
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17 Arrhenius, Svante August 
 Archimedes of Syracuse (287–212 bc ) A Greek mathematician and philosopher cred-
ited with the principles of levers, the Archimedean screw as a pump, and a method of 
successive approximations which allowed him to determine the value of π to a good ap-
proximation. King Hiero is said to have asked Archimedes to check if a crown was pure 
gold throughout or contained a cheap alloy. While in a public bath and pondering on 
how to do this without damage to the crown, Archimedes is supposed to have suddenly 
thought of the possibility of immersing it in water and checking its density by way of dis-
placement, and to have been so excited that he ran naked through the streets shouting 
‘Eureka! Eureka! I have found it! I have found it!’ He was killed by a soldier in the Roman 
siege of Syracuse.
 Archimedes’ principle A principle that states when a body floats it displaces a weight of 
liquid equal to its own weight. The principle was not stated by Archimedes but is connected 
to his discoveries in hydrostatics. When a body is partially or totally immersed in a liquid, 
there is an upthrust on the body equal to the weight of the liquid displaced by the body.
 area The extent of a plane figure or surface. The area of a rectangle is the product of the 
length and base. The area of a circle of diameter d is πd2 4/ . The SI unit is m 2 .
 Argand diagram A graphical way of representing complex numbers in the form 
z x jy= + in which real and imaginary parts of the complex number are the x and y axes, 
respectively ( see Fig. 2 ). The modulus is the distance z and the angle of z is the argument. 
 It is named after Swiss mathematician Jean-Robert Argand (1768–1822) and is useful in 
understanding the stability of controlled processes. 
0 x (real)
|z|
A
y 
(i
m
a
g
in
a
ry
)
 Fig. 2 
 Arrhenius, Svante August ( 1859–1927 ) A Swedish physicist and chemist who did 
fundamental work on physical chemistry. He worked with * van’t Hoff in Amsterdam and 
proposed a theory of activated molecules and established a connection between rate of 
reaction and absolute temperature. He also developed a theory for electrolytic dissocia-
tion based on van’t Hoff’s results and stated that any acid, base, or salt dissolved in water is 
partly split up into positively and negatively charged ions, and that they move in opposite 
directions on electrolysis. He was awarded a Nobel Prize for Chemistry in 1903.
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• Ofi cial website of the Nobel Prize organization, with a transcript of Arrhenius’ lecture of 
1903.
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Arrhenius equation 18
 Arrhenius equation An equation that represents the effect of temperature on the vel-
ocity of a chemical reaction expressed as: 
 
d k
dT
E
RT
k Aea
E
RT
a ln
 = =
−
2
or 
 where k is the * rate constant for the reaction, E 
a 
 is the * activation energy , R is the gas 
 constant, T is the absolute temperature and A is frequency factor. An Arrhenius plot of ln k 
against 1/T gives a straight line of slope –E/R and is valid for a large number of chemi-
cal reactions. It is named after Swedish chemist and physicist Svante August * Arrhenius 
(1859–1927).
 aseptic A condition in which all contaminating microorganisms are eliminated, not pres-
ent, or allowed to reproduce. Substances that provide aseptic conditions are known as an-
tiseptics. Aseptic processing involves ensuring sterility and therefore a process that is free 
from microbial contamination. It is used in the packaging of foods, pharmaceuticals, and 
medical products. Sterility is achieved using a flash-heating process and the product pack-
aged into aseptic containers. The container is required to be robust and provide a tight seal 
against outside contamination sources. The container and its contents have the benefit of 
not requiring refrigeration. Compare sterility .
 ash The non-volatile products and residues that remain after a combustion process. * Elec-
trostatic precipitators are used to remove ash particles from flue gas streams.
 ASME See american society of mechanical engineers .
 aspect ratio The ratio of the height to width or diameter of an item of process plant 
equipment such as a column or storage tank.
 asphyxia A state of unconsciousness as the result of anoxia or * hypoxia and increased 
carbon dioxide in blood and tissue. See suffocation .
 assay An analytical procedure used in the laboratory for assessing a sample of something 
either qualitatively or quantitatively in terms of an amount of a substance, its composi-
tion, or some other entity under investigation. Biochemical assays of samples taken from 
bioreactors that often feature complex mixtures involve procedures for determining cell 
content, substrate utilization, and product formation. The complexity of the samples that 
require various prescribed steps to be followed does not permit conventional forms of 
chemical analysis such as titration.
 association The grouping together of atoms or molecules often in the vapour state or in 
solution to form conglomerates of high molecular weight. Compare dissociation .
 assumption A statement that is used in order to simplify a problem, in reaching a solu-
tion, or where the full understanding of the problem is not actually known. Assumptions 
arise in all forms of chemical engineering. For example, in distillation, liquid and vapour 
are often assumed as being at equilibrium on a theoretical stage; a reaction mechanism 
may assume no side reactions; the flow from a vessel may assume no vortex formation or a 
constant discharge coefficient.
 ASTM See american society for testing materials .
 asymptote A straight line that is closely approached by a curve so that the perpendicular 
distance between them decreases to zero at an infinite distance from the origin.
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19 atomic nucleus 
 ATEX An EU directive that describes the work that may be safely carried out in an explosive 
atmosphere. The areas or zones in a process plant are classified according to the type of 
hazards, the location, and size, and the likelihood of an explosion. It is applied tomining 
operations, offshore processing, petrochemical plants, and flour mills, where potentially 
explosive atmospheres may exist. The name is derived from the French title for the EU 
 directive Appareils et systèmes de protection pour les atmosphères explosibles. 
A
• Ofi cial website of the Health and Safety Executive, UK, outlining information on ATEX and 
explosive atmospheres.
 atmolysis The separation of a mixture of gases by diffusion through a porous membrane 
such as hollow fibres. Each gas in the mixture has a different rate of diffusion, allowing them 
to be separated.
 atmosphere A layer of gases of largely oxygen (21 per cent) and nitrogen (79 per cent) 
surrounding the Earth’s surface which comprises the troposphere, stratosphere, and iono-
sphere and traceable to an altitude of around 800 km. The barometric pressure varies with 
altitude with * standard atmospheric pressure at sea level being taken to be 101,325 Pa or 
1,013 mbar.
 atom The smallest particle of an element that can exist and which can take part in a 
chemical reaction and cannot be chemically divided any further into smaller parts. It is 
identifiable as that element by its nucleus. The nucleus contains neutrons and protons and 
is surrounded by a cloud of orbiting electrons. The number of electrons equals the number 
of protons such that the overall charge is zero.
 atom balance A material balance based on the number of atoms of specified elements.
 atomic bomb A nuclear weapon whose explosive force is due to the energy released 
through the process of nuclear fission. It involves bringing together a mass of fissile ma-
terial sufficient to result in a chain reaction that proceeds explosively. Uranium-235 and 
plutonium-239 are examples of fissile material used in nuclear weapons. The explosive 
force of nuclear weapons is quoted in kilotonnes or megatonnes of * TNT equivalents . 
The atomic bombs that were dropped on Hiroshima (uranium-235 bomb) and Nagasaki 
(plutonium-239 bomb) had the explosive energy equivalent to 13 and 22 kilotonnes of 
TNT, respectively.
 atomic energy See nuclear energy .
 atomicity The state of being made up of atoms and is the number of atoms in molecules. 
For example, carbon dioxide (CO 
2
 ) has an atomicity of 3; hexane (C 
6
 H 
14
 ) has an atomicity 
of 20, etc.
 atomic mass The mass of an isotope of an element expressed in * atomic mass units . It is 
short for * relative atomic mass .
 atomic mass unit (a.m.u.) A unit of mass used to express atomic and molecular 
weights. It is equal to one twelfth of the mass of an atom of carbon-12 and is equivalent 
to 1.66 × 10 –27 kg.
 atomic nucleus See nucleus .
a
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atomic number 20
 atomic number The number of protons in an atomic nucleus. The classification of ele-
ments is based on the increasing order of atomic number.
 atomic pile An early name for a * nuclear reactor that used graphite as the * moderator . See 
 windscale nuclear accident .
 atomic power An alternative name for * nuclear power .
 atomic volume The * relative atomic mass of an element divided by its density.
 atomic weight See relative atomic mass .
 atomization The creation of very small droplets of a liquid within a gas. The droplets 
may range in size from 10 micrometres to 1 millimetre and consequently have a very high 
surface area, thereby permitting rapid chemical reaction, drying, heat, and mass transfer. 
Atomization is particularly useful for fuels in combustion processes and for drying or dehy-
dration of liquid products in spray dryers using an * atomizer .
 atomizer A device used in the process of atomization to produce very small droplets of a 
liquid within a gas. Such small droplets can be produced by forcing a liquid through a very 
small aperture under high pressure or by contacting the liquid with a high-speed rotating 
plate or disc.
 auriferous A rock or ore containing gold.
 austenitic stainless steel An alloy of iron that contains at least 8 per cent nickel and 
18 per cent chromium. It is noted for its very good corrosion resistance, heat resistance, 
and creep resistance, and is also non-magnetic. It is used extensively for process pipes and 
vessels.
 autocatalysis A catalyzed chemical reaction in which one of the products is the catalyst 
for the reaction. The chemical reaction starts slowly as the catalyst is formed and continues 
rapidly until the point when the reactants are depleted.
 autoclave A sealed and heated thick-walled pressure vessel used for the thermal sterili-
zation of biological agents and tinned food products using steam. It is also used for carrying 
out chemical reactions at elevated temperatures. 
 autoignition temperature The temperature at which a material ignites in air or some 
other oxidant at a specified pressure without the aid of a spark or flame. The minimum 
autoignition temperature is determined by an * ASTM test method. It is also known as the 
 autonomous ignition temperature . 
 automatic control See feedback control .
 autoradiolysis The dissociation of molecules contained within a substance or mixture 
through * ionizing radiation arising from radioactive materials such as in highly active * nu-
clear waste .
 autothermal A system, process, or reaction that is completely self-sufficient in terms of 
its energy requirements. Some * exothermic reactions are autothermal. Some * anaerobic 
digesters are operated in this way in which the methane liberated is used to fuel the process.
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21 azeotrope 
 average velocity Also known as the * mean velocity , it is the total volumetric flow rate 
of a fluid per unit flow area. It is a useful parameter particularly where there may be local 
variations in velocity and hence flow across a flow area due to the effects of turbulence or 
obstructions in a pipeline, duct, or stack. The SI units are m s –1 .
 aviation gasoline A hydrocarbon fuel produced in petrochemical refineries. It is used 
by aircraft with piston engines. It has a high * octane rating and more closely resembles 
motor gasoline or petrol than diesel fuel. See jet fuel .
 Avogadro, Amedeo (1776–1856) An Italian chemist and physicist who provided Avoga-
dro’s law as a way of calculating molecular weights from vapour densities. He was educated 
and graduated in ecclesiastical law; however, he had a keen interest in the natural sciences 
and received private tuition in physics and mathematics. He published his hypothesis, 
known now as * Avogadro’s law , while working as a schoolteacher. He was appointed to 
the first chair in mathematical physics at Turin University in 1820. The importance of his 
work was first recognized by the Italian chemist Stanislao Cannizzarro (1826–1910) in 1858, 
shortly after Avogadro’s death.
 Avogadro’s constant (Symbol N 
A
 ) The number of atoms in one mole of a substance. 
It has the value of 6.022 1367(36) × 10 23 and was formerly known as Avogadro’s number . 
 Avogadro’s law A law that states that equal volumes of gases at the same temperature 
and pressure contain the same number of molecules. This was first stated as a hypothesis 
by the Italian chemist and physicist Amedeo * Avogadro (1776–1856) in 1811. However, 
this law was not generally accepted until after his death when the Italian chemist Stanislao 
 Cannizzaro was able to explain why there